A Metropolitan police officer who allegedly stamped on an arrested teenager, leaving him struggling to breathe, has been accused of trying to "smear" the youth as he denied the charges.

Prosecutors claim PC Joseph Harrington, 29, assaulted the boy, then 15, while he was being held in custody days after riots swept the country in 2011.

On Wednesday he insisted he restrained the boy only after he became aggressive and suggested the youngster had a history of making false allegations.

Harrington said the youth, who cannot be named for legal reasons, left him in "fear" for his safety during a heated confrontation in a caged cell at Forest Gate police station.

The teenager has told the court how an officer stamped on him then "dropped his whole body" on to his back after apparently recognising him from a confrontation a day or two before.

On that occasion, he claims, Harrington put his hands around his neck.

Southwark crown court has heard how the teenager was arrested for a separate matter on 11 August and was taken from Stratford police station, east London, to Forest Gate police station.

He was in the caged area near the custody suite when Harrington walked in and made reference to their previous meeting.

The 17-year-old said: "I laughed at him. I thought: 'I'm already arrested for something I haven't done, I'm handcuffed, in a police cage, what could get worse?"'

He was said to have become verbally abusive before an older policeman grabbed him by his jacket, which ripped. Officers dragged him across the floor before Harrington allegedly stamped on his back, he told the court.

Giving evidence in his defence, Harrington dismissed suggestions that he unnecessarily resorted to force.

"I was sticking to my training and what my experience told me I needed to do," he said. He told the court officers had endured a "very tiring" few days during the riots, when police were subjected to days of abuse.